In the manufacture of steels it has for many years been practice to treat melts with stirring gases in a secondary metallurgical processing. Metallurgical vessels in which such treatments are performed are ladles, furnaces and converters. In the making of steel in electric arc furnaces it is also known to stir up the molten metal, thereby purging and intensifying metallurgical reactions by introducing inert gases into the molten metal through nozzles being disposed in the furnace hearth or bottom. For example, in the process disclosed in German Offenlegungsschrift 1 583 221 gas-permeable refractory bricks are disposed in the area below the position of the electrodes in the furnace hearth. However, gas-permeable refractory bricks have the disadvantage of a short service life in comparison with the adjoining refractory hearth lining.
In the electric arc furnace disclosed in the published European patent application 0 200 405 the melt process can be boosted by introducing a stirring gas into the melt through a small tube in the furnace bottom. However, no concrete details are mentioned concerning the construction of the small tube or the furnace bottom. The European patent application 0 240 998 discloses a small tube extending through both the permanent lining and the wear lining. The small tube is therefore directly exposed to wear by the molten metal melt in the furnace.
In the case of stirring (purging) in an electric arc furnace other important aspects are the geometrical arrangement of the small gas tubes in the furnace bottom and the formation of the stirring spot in view of the circulation of the molten metal to be achieved and the need for the arcs to be adjustable to burn quietly and uniformly. The construction of the stirring spot depends on the quantity of the gas and the kind of distribution of the gas bubbles emerging at the boundary surface between the furnace bottom and the molten metal.
In the German Journal "Stahl und Eisen" 106, 1986, No. 19, pages 1003 to 1005, the introduction of argon as a stirring gas into the melt through a spirally shaped small tube disposed in the constantly refilled tapping channel of the furnace is described. This is an experimental arrangement stated to have the disadvantage that the small stirring gas tube extending loosely through the filling material leads to a weakening of the tapping filling. This publication also states that it was a troublesome and time-wasting operation to have to reintroduce the small stirring gas tubes every time into the tapping channel, so that a stirring set was developed. However, the publication gives no concrete details of the construction of the stirring set.
German patent 3 318 422 discloses a stirring gas arrangement in the wall of a melt-containing vassel, wherein a small stirring gas tube is mounted for longitudinal displacement in the vessel wall The main objective of this arrangement is to enable the stirring gas arrangement to be restored to operation in a simple and quick manner by moving up the small stirring gas tube when the arrangement has become unusable due to molten metal freezing in the tube. When this arrangement is used in the hearth/bottom in which the wear lining consists of a refractory ramming mass, it must be expected that the emerging gas over the burnt-back small stirring gas tube will cause funnel shaped washed out places in the rammed hearth/bottom, so that it is of low durability. Another disadvantage will be the deficient sealing of the advanceable small stirring gas tube in the bottom brick, resulting in a leakage and the loss of a proportion of the stirring gas.
It is an object of the invention to provide a metallurgical vessel, more particularly an electric arc furnace, in which both the gas stirring device and also the lining of the furnace hearth or bottom have a long service life. Furthermore, with an improved melt stirring effect, the large area introduction of the stirring gas into the molten metal achieves a formation of the stirring spot which ensures that the arcs burn uniformly and quietly.